A typical nuclear plant has its upper pool in a reactor building and the lower pool of a fuel-element storage unit. The reactor can be a boiling-water reactor, for example, or also a pressurized-water reactor. Fuel elements are transferred between the pools by a system or apparatus comprising
a conveyor tube connecting the upper and the lower pool and extending at an acute angle to the vertical,
one or more transport baskets into each of which at least one of the fuel elements can be placed for transfer through the conveyor tube,
an upper transfer device in the upper pool for loading fuel elements into and unloading them from the transport baskets, and/or
a lower transfer device in the lower pool for loading fuel elements into and unloading them from the transport baskets.
In a nuclear plant, replacement and transfer of fuel elements has particular importance in practice. In this connection, fuel elements generally consist of a bundle of individual fuel rods, and the fuel element itself is equipped with a handle or the like so that it can be transported using suitable machines, for example in order to set it into the reactor vessel or remove it from the reactor vessel. Thus, spent fuel elements, in particular, must be removed from the reactor vessel and transported to a fuel-element storage unit, for example. Conversely, fresh fuel elements must be loaded into the reactor vessel. In practice, it is usual to fill the upper pool in the reactor building during the fuel element exchange, so that the fuel elements are transported in liquid (water). They are taken out of the open reactor vessel using a handler that can be moved above the reactor vessel and moved into the upper pool and temporarily stored, if necessary, in a buffer pool/cooling pond. From the upper pool, the fuel elements must be transported to a fuel-element storage unit, for example, using a transfer system, the storage unit also having a (lower) pool, the upper pool (for example in the reactor building) and the lower pool (for example in the fuel-element storage unit) being filled to a different liquid level, independent of one another. Transport using the transfer system takes place between these two pools through a conveyor tube mounted at an angle to the vertical. Such transfer systems are basically known from practice. In this connection, an effort is made to keep the time expenditure for a fuel element exchange as short as possible in order to reduce interruptions in the power operation of the reactor as much as possible. The reduction in the time required for the fuel element exchange has particular importance from an economic point of view.
In a transfer system for fuel elements of a nuclear reactor facility known from U.S. Pat. No. 3,952,885 the fuel elements are transported through a conveyor tube oriented at an angle to the vertical. The conveyor tube leads through the safety sheath that encloses the pressurized reactor vessel of a pressurized-water reactor in a gas-tight manner. The inner pool and the outer pool are filled to the same liquid level during the fuel element exchange, so that no blocking measures in the region of the conveyor tube are necessary during transfer of the fuel elements. Transport takes place using a cable hoist and using a carriage that has two chambers, of which one accommodates a fresh fuel element for the trip there, and the other a spent fuel element for the return trip. In this connection, the carriage can pivot from a vertical transfer position into a horizontal or angled transport position. The known transfer system exclusively serves for transfer of fuel elements between two pools filled to the same liquid level.
The same holds true for the transfer system known from U.S. Pat. No. 4,096,031, with which fuel elements are transported directly between the reactor vessel and a storage container mounted directly next to it, where the reactor vessel and the storage container are connected with one another by a transport tube that extends at an acute angle to the vertical. Transfer devices are provided at the end of this conveyor tube that can pivot the fuel elements from an angled transport position into a vertical transfer position. Transfer through the conveyor tube takes place using a cable hoist that directly grips the fuel element with a grab.
U.S. Pat. No. 3,058,900 describes a charging apparatus for nuclear reactors in which fuel elements are transported directly between the reactor vessel and a channel that runs horizontally below the reactor vessel, the reactor vessel and the channel being connected with one another by a tube that is oriented at an angle to the vertical. For transport, a fuel element is inserted into a cartridge that can be transported through the tube, the cartridge bing provided at the top with a handle or the like so that it can be gripped by a tool. The tube can be closed off completely using a blocking element. This blocking element is opened when a fuel element exchange takes place by means of the charging apparatus.
A loading and unloading apparatus for fuel elements is known from U.S. Pat. No. 4,202,729 in which the apparatus sits on the top of the reactor vessel and has two ramps extending at an angle to the vertical in opposite directions and between which the fuel elements can pivot using a pivoting apparatus. An apparatus having a similar construction is known from U.S. Pat. No. 4,440,718.